This invention relates to heavy duty industrial, mining and general purpose cutting tools, and more particularly to cutting tools of the type having a bonded abrasive element forming the primary working element.
Decorative coatings for appearance, and wear resistant, hard surface coatings for protecting metal substrates against corrosion, thermal shock and the like are both well known in the prior art. The hard surfacing of a certain class of tools having a continuous cutting blade (such as agricultural implements, for instance) has been employed to form a primary, high hardness, working edge that is "self sharpening" during use in that wear of the hard surface material and tool blade continues to present a sharp primary working edge throughout. Such hard surfacing conventionally is accomplished by fusion in high temperature furnaces using high iron content, metal alloys, see Alessi U.S. Pat. No. 3,600,201. Other techniques for hard surfacing metal substrates to provide wear resistant, anti-corrosive metal coatings include flame or plasma spraying, detonation gun applications and the like as discussed in Patel U.S. Pat. No. 4,075,371 and Weatherly U.S. Pat. No. 4,173,685. Moore U.S. Pat. No. 2,857,292 teaches the application of high nickel content surface coatings to protect airplane engine parts and other ferrous alloys from corrosion, weathering and other deteriorating agents. The prior art is devoid, however, of any showing of the hard surfacing of heavy duty "cutting" tools employing abrasive inserts as the principal working element and in which the surface coating itself substantially enhances the working life of the tool by maintaining its integrity during normal wear of the abrasive insert workpiece. In the past a wide variety of industrial or general purpose cutting tools have been designed for numerous "cutting" functions including trenching, boring, drilling, sawing, and crushing. Typical cutting tools may use a single or continuous cutting surface or edge, but more frequently employ a plurality of discrete, replaceable cutting elements or bits either sequentially and angularly arranged on a chain, wheel, caisson or like continuous carrier or being disposed in a predetermined sequence or pattern on a rotary bit or auger of some type. A typical class of cutting tools, to which the present invention is particularly applicable, involves industrial mining equipment utilizing a series of sequentially spaced and angularly disposed "pencil" drill bits of the type disclosed herein, which have carbide or like abrasive inserts or tips to perform the primary cutting function.
In these mining tools the abrasive insert tip conventionally is brazed to the main body of the bit by silver solder to secure a solid bond that will withstand the large striking or impact forces thereon as the bit is carried into striking "cutting" engagement with the work product, such as coal, mineral ores or the like. The high cost, today, of silver solder has caused a search for acceptable, alternative brazing compounds. The industry now extensively uses bronze (or copper) brazing, but this requires substantially high brazing temperatures than silver and above the temperature at which temper (Rockwell hardness) of tool steels is lost, thereby influencing the choice of air or oil hardening processes for re-tempering the tool steel. Obviously, the tempering of tool steels has a direct bearing on metal stress, thermal shock and undetectable fracture lines so that a large number of abrasive inserts and/or bit bases are cracked or otherwise weakened in production with the end result of damage or loss of mining bits and increased production costs due to downtime and replacement expenses during mining operations.
Another problem encountered in such industrial mining equipment is that, with the advent of higher speed equipment and heavier impact forces, rapid tool wear and breakage has appreciably increased thereby causing re-design to heavier, bulkier bit configurations to support the carbide inserts and withstand these forces. However, more massive bits create higher dust levels that are more difficult to control under the stringent mining safety regulations, and non-productive downtime in operations frequently is mandated merely to bring dust levels under specified concentrations.